The invention relates to an aberration correcting device for an electron microscope, and to an electron microscope comprising such an aberration correcting device.
Electron lenses as used in electron microscopes intrinsically suffer from a positive spherical and chromatic aberration. These aberrations limit the resolution of said electron microscopes. It is possible to at least partially compensate for these aberrations by introducing other components in the electron microscope column that provide a negative spherical and chromatic aberration.
The United States Patent Application US2007/0200070 discloses an aberration-correcting microscopy instrument comprising an electron mirror configured for correction of one or more aberrations. The instrument is provided with a magnetic deflector which deflects the incoming electron beam, coming from the electron gun, over an angle of 90 degrees towards the electron mirror. The electron mirror is arranged such that the mirror axis is arranged perpendicular to the optical axis of the incoming electron beam. The electron mirror corrects for both chromatic and spherical aberration, and reflects the corrected electron beam back to the magnetic deflector. The reflected beam is again deflected over an angle of 90 degrees by the magnetic deflector, which returns the reflected beam onto the same optical axis as the incoming beam.
A disadvantage of the known system is that the electron beam experiences a focusing effect by the magnetic field of the magnetic deflector. Moreover the focusing effect in the plane of deflection is different from the focusing effect in a plane perpendicular to the plane of deflection. In addition, the actual deflection angle of the electrons in the electron beam depends on the energy of the electrons, which causes dispersion for electron beams comprising electrons with different energies. These effects deteriorate the resolution of the electron microscope, and must be carefully compensated in the aberration-correcting microscopy instrument in order to improve the resolution in the microscope. This leads to complex designs with many magnetic coils and high demands on the accuracy of the manufacturing.
An alternative system for removing distortions from an electron image is described in U.S. Pat. No. 5,321,262, which discloses an image band pass filter for a photoelectron spectromicroscope. The image band pass filter comprises two stages, a first stage comprising a region in which a beam of imaging electrons is deflected in the crossed electrostatic and magnetic field between a succession of electron mirrors which are designed to remove electrons of a certain energy and reflect those of a particular energy which are to form the electron image. The beam from the first stage is then transferred to a second stage through a high pass filter into a region of the second stage and then deflected in an electrostatic and magnetic cross field along a path corresponding to a restoration of the displacement of the original beam in the first stage from its original path of travel. In the first and second stage, the electrostatic field of the cross field is designed to be non-uniform to control motion of the electrons in the cross field and to correct for distortion of the image in the output beam. Accordingly, the image band pass filter as described in U.S. Pat. No. 5,321,262 is designed to remove distortion of the electron image due to the use of inhomogeneous electrostatic fields of the deflecting crossed electrostatic or magnetic field employed.
It is an object of the present invention to ameliorate one or more of these problems or to at least provide an alternative aberration correcting device for an electron microscope.